In the production of flexible circuitry, a conventional practice is to provide a relatively thin web or sheet of material with circuitry on one or both sides. The web of material is then used to make various electrical connections utilizing its flexibility. As the number of connections increase, it is increasingly desirable to have circuitry on both sides of the web of material, and hence this has become a significant product, i.e., a product with circuitry on both sides thereof. As the technology advances, the density of the circuitry increases, i.e., the width of the lines, decrease and the spacing between the lines decreases, thus allowing more circuitry to be provided in a given surface area. Also typically it is necessary for the circuitry or circuit traces on both sides of the flexible material to be in relatively close alignment with each other, i.e., the circuitry on one side must be in close alignment with the circuitry on the opposite side thereof. Hence, as the density of the circuitry increases, the tolerance for misalignment decreases, thus requiring closer and closer tolerances between the circuitry on opposite sides.
A photolithographic process is the preferred process for forming circuitry on opposite sides of the web. In such a process, photoresist material is coated on both sides of the web and exposed to actinic radiation in the desired pattern, followed by a development of the exposed photoresist to reveal or transfer the pattern to the substrate. Following this, the circuitry is formed by conventional additive or subtractive plating processes which are not the subject of the present invention. Thus, it becomes necessary in this very early stage in the process to be sure that the exposed patterns are aligned within the allowable tolerances to provide their required degree of alignment. Such two-sided exposure has been attempted at the same station at the same time; however, this poses significant problems of attempting to expose both sides while chucking the material. However, attempts to do the two-sided exposure at separate stations at separate times has not proved entirely satisfactory as the circuit density increases due to the difficulty of providing the necessary alignment of the pattern on the second side of the web to the pattern previously exposed on the first side.
Therefore, it is a principal object of the present invention to provide a method and apparatus for exposing opposite sides of a web of material to actinic radiation for a lithographic process at spaced stations simultaneously with a close alignment of the exposed patterns on both sides necessary for fine circuitry formation.